[openssl-users] Rejecting SHA-1 certificates

Niklas Keller me at kelunik.com
Mon Jul 10 17:12:08 UTC 2017

> > On Jul 10, 2017, at 3:45 AM, Niklas Keller <me at kelunik.com> wrote:
> >
> >
> > What's the best way / a working way to reject weak signature schemes in
> OpenSSL 1.0.{1,2}?
> Most CAs have stopped issuing SHA-1 certificates.  Any old ones will
> expire over the
> next year or two.  While Google has demonstrated a SHA-1 collision, that
> proof of
> concept is far from a practical attack.

Actually they should already be expired, all major browsers will reject
them already, even Edge.

> The simplest solution is to let the CAs solve the problem as SHA-1
> certificates fade
> out of the picture.  You can if you wish leave out from the set of trusted
> roots any
> CAs that have not yet stopped issuing SHA-1 certificates.

CAs can't solve the problem that we accept certificates with weak

> You can of course implement a verify callback that inspects each
> certificate in the
> chain, and triggers an error when its signature is SHA-1 and it is not the
> last one
> in the chain.  This requires keeping some state attached to the X509 store
> context,
> and I don't think is worth the effort.

It's very well worth the effort, otherwise there's a security issue,
because certificates can be forged.

Regards, Niklas

> See code involving "TLScontext_index" in:
> https://github.com/vdukhovni/postfix/blob/master/postfix/
> src/tls/tls_client.c#L318
> https://github.com/vdukhovni/postfix/blob/master/postfix/
> src/tls/tls_client.c#L942
> https://github.com/vdukhovni/postfix/blob/master/postfix/
> src/tls/tls_verify.c#L163
> With such a context, you can keep track of the maximum depth seen by the
> callback,
> and reject SHA-1 at lower depths.  I do not recommend doing this.
> --
>         Viktor.
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